This disclosure relates generally to tracking systems that use magnetic fields to determine the position and orientation of an object, such as systems used for tracking instruments and devices during surgical interventions and other medical procedures. More particularly, this disclosure relates to a system and method of tracking including a coil transmitter having two coils operating at about the same frequency.
Tracking systems have been used in various industries and applications to provide position information relating to objects. For example, electromagnetic tracking may be useful in aviation applications, motion sensing applications, and medical applications. In medical applications, tracking systems have been used to provide an operator (e.g., a physician) with information to assist in the precise and rapid positioning of a medical device located in or near a patient's body. In general, an image may be displayed on a monitor to provide positioning information to an operator. The image may include a visualization of the patient's anatomy with an icon on the image representing the device. As the device is positioned with respect to the patient's body, the displayed image is updated to reflect the correct device coordinates. The base image of the patient's anatomy may be generated either prior to, or during, the medical procedure. For example, any suitable medical imaging technique, such as X-ray, computed tomography (CT), magnetic resonance imaging (MRI), positron emission tomography (PET), and ultrasound, may be utilized to provide the base image displayed during tracking. The combination of the base image and the representation of the tracked device provide positioning information that allows a medical practitioner to manipulate a device to a desired position and/or associate information gathered to a precise location.
To determine device location, tracking systems may utilize a method of electromagnetic (EM) field generation and detection. Using this method, at least one magnetic field is generated from one or more EM sensors (e.g., EM field generators or transmitters), and the magnetic fields are detected by one or more complementary EM sensors (e.g., EM receivers). In such a system the EM field may be detected by measuring the mutual inductance between the EM sensors and the complementary EM sensors. The measured values may then be processed to resolve a position and/or orientation of the EM sensors relative to one another. For example, an electromagnetic tracking system may include an EM sensor mounted at the operative end of a device and a complementary EM sensor fixed in a known position. When the EM sensor generates a magnetic field, a voltage indicative of the mutual inductance may be induced across the complementary EM sensor. The signal may be sensed and transmitted to a processor for processing. Processing may then use the measured voltage signal indicative of mutual inductance to determine the position and orientation of the EM sensors relative to one another (e.g., the X, Y and Z coordinates, as well as the roll, pitch and yaw angles).
Generally electromagnetic tracking systems contain EM sensors that include an array of one or more EM transmitter coils and an array of one or more EM receiver coils. Preferably, the mutual inductance between the two coils may be measured without inaccuracies. However, when measuring the mutual inductance between the transmitter and receiver coils, electrically conductive materials in the vicinity of the transmitters and receivers may distort the electromagnetic fields generated by the transmitter. For example, a nearby metal instrument may create distortions in the magnetic field. These distortions may lead to inaccuracies in tracking position and orientation.
As an additional consideration, electromagnetic tracking systems may be limited by the number of degrees of freedom they are able to track. In general, the number of degrees of freedom that an electromagnetic tracking system is able to track and resolve depends on the number of receiving and transmitting coils in the system. For example, a system consisting of a single transmitting coil and multiple receiver coils may be tracked in only five degrees of freedom. As will be appreciated, this is because a dipole coil is uniform about its axis and, therefore, processing cannot resolve the rotational orientation of the coil transmitter.
Accordingly, there is a desire to provide an electromagnetic field tracking system, wherein EM sensors are configured to limit the impact of magnetic field distortions. There is also a desire to provide a system configured to track in six degrees of freedom.